1. Field of the Invention
The present invention relates to a speaker telephone, and particularly to a circuit for detecting the presence of audio incoming from the telephone line while the user is speaking. The application relates to the inventor's Disclosure Document No. 087,715 filed in the U.S. Patent Office on Jan. 29, 1980.
2. Description of the Prior Art
In a speaker telephone, the user's voice is picked up by a microphone and amplified for transmission to the telephone line. When the other party begins to speak, appropriate detection circuitry disables microphone transmission and enables amplification and loudspeaker reproduction of the incoming audio.
Implementation of the incoming audio detection circuitry poses certain difficult problems. If the user is talking at the same time that the other party begins to speak, both outgoing and incoming audio signals simultaneously are present at the telephone line connections to the speaker telephone. Typically, the outgoing signal is 15 db louder than the incoming signal. Thus the detection circuitry must distinguish between two speech signals of significantly different magnitude that are both present across the same pair of terminals.
In some prior art speaker telephones the problem is avoided by detecting the incoming signal only during pauses in the user's speech, when no outgoing signal is present. In such systems, the other party cannot effectively interrupt while the user is talking, and such party may experience an objectionable delay before the user recognizes that he has begun to speak.
Another approach to incoming signal detection involves the use of a directional coupler. A directional coupler works by reason of polarity. Typically, two transformers are connected to the telephone line in such a way that there is a polarity sense which permits separation of outgoing and incoming signals. However, such a directional coupler arrangement is very much dependent on telephone line impedance. In practice, different telephone lines exhibit different impedances, and the impedance of a particular line may change from time to time, both in resistive and reactive components. Should this occur, the ability of a directional coupler to separate incoming and outgoing signals may be substantially degraded. Thus the use of a directional coupler is not preferred from a standpoint of reliability.
In other speaker telephones a high frequency sampling method is used to detect incoming speech while the user is talking. Complex circuitry is required to sample the line at very rapid intervals with the objective of detecting a signal at the telephone line at an instant when no voice is picked up by the microphone.
A principal objective of the present invention is to provide a cost effective, simple circuit for detecting an incoming signal while the user of a speaker telephone is talking.